Signalling in a telecommunications system

ABSTRACT

Method for transmitting signaling information in a telecommunications network between a pair of application parts, the method comprising the steps of: generating signaling information at a first application part, including addressing information associated with the second application part, passing the signaling Information from the application part to a Transaction Capabilities Application Part (TCAP); passing the signaling information from the TCAP to an adaptation layer arranged to determine an IP address and port number associated with the addressing information; passing the signaling Information and the IP address and port number to an Internet Protocol (IP) part and transmitting the signaling information over an IP network to the destination IP address in one or more IP datagrams; decapsulating the signaling information at the destination associated with the IP address and at an adaptation layer identified by the port number; and routing the signaling information to the second application part.

FIELD OF THE INVENTION

The present invention relates to signalling in a telecommunicationssystem and in particular, though not necessarily, to the transmission ofsignalling data in a Public Land Mobile Network.

BACKGROUND TO THE INVENTION

In a telecommunications system, signalling equipment and signallingchannels are required for the exchange of information between systemelements or nodes. In particular, this internode signalling informs thenodes of what is to be performed when a telephone or data call is to beset up or released in so-called “circuit-switched” connections.Signalling is also often used to communicate information on the statusof the system and of individual subscribers.

Modern telecommunications systems now largely make use of Common ChannelSignalling (CCS) whereby signalling information is transmitted on one ormore dedicated signalling channels, distinct from the channels used tocarry actual user information (e.g. voice or data). An important featureof CCS is that the same signalling system may support services in avariety of existing telecommunications networks, e.g. Public SwitchedTelephone Network (PSTN), Integrated Services Digital Network (ISDN),and Public Land Mobile Networks (PLMN), as well as proposed futureprotocols such as B-ISDN, enhancing greatly the interoperability ofnetworks supporting different protocols.

Currently, the predominant CCS is known as Signalling System Number 7(SS7), defined in the ITU-T (International TelecommunicationsUnion-Technical) recommendations starting with Q.700. SS7 is a packetswitched system occupying one time slot per frame of the Time DivisionMultiple Access (TDMA) E.1 or T.1 transmission formats (the other timeslots being available for user voice or data information). Individualsignalling message packets (referred to as Message Signalling Units orMSUs) are associated with respective individual telephone calls. As onlya relatively small amount of signalling information is associated with asingle telephone call, a single SS7 channel is able to handle allsignalling between two network nodes (termed “signalling points”) forseveral thousands of calls. It is noted that the route taken by an MSUin the SS7 network may be the same as that over which the associatedtelephone call is established, or it may be different.

As already noted, SS7 (along with other CCS systems) is able to supporta number of different telecommunications networks (e.g. PSTN, ISDN,PLMN). In signal processing terms, SS7 comprises a Message Transfer Part(MTP) which deals with the physical transfer of signalling informationover the signalling network (MTP layer 1), message formatting, errordetection and correction, etc (MTP layer 2), and message routing (MTPlayer 3). SS7 also comprises user parts and application parts whichallow several “users” (i.e. ISDN User Part, Telephony User Part, MobileApplication Part, etc) to send signals in the same signalling network.

FIG. 1 illustrates a PLMN (GSM) having a Gateway Mobile Switching Centre(GMSC) 1 which provides an interface for the PLMN to “foreign” networkssuch as ISDNs, PSTNs, and other PLMNs. An exemplary Mobile Station (MS),registered to the PLMN, is indicated by the reference numeral 2. Anumber of exemplary nodes within the PLMN are also shown including: theGMSC 1; a Mobile Switching Centre (MSC) 3; a Base Station Controller(BSC) 4; a Base Transceiver Station (BTS) 5; an Intelligent Network node(IN) 6; a Visitor Location Register (VLR) 7; a Home Location Register(HLR) 8; an Authentication Centre (AUC) 9; and an Equipment IdentityRegister (EIR) 10. A more complete description of a PLMN, and otheraspects of telecommunication networks, is given in “UnderstandingTelecommunications”, vols. 1 & 2, Studentlitteratur, Lund, Sweden (ISBN91-44-00214-9).

FIG. 1 indicates the various signalling interfaces which are used tosignal between the network nodes detailed above. These interfacesinclude: the ISUP/TUP interfaces between the GMSC 1 and the foreignnetworks; the INAP interface used between the MSC 3 and IntelligentNetwork (IN) nodes; the MAP interface used between the MSC/GMSC 1,3 andPLMN specific nodes (VLR, HLR, AUC, EIR) 6-10; the BSSMAP used betweenthe MSC 3 and the BSC 4; and the Abis interface between the BSC 4 andthe BTS 5. Conventionally, all of these interfaces serve as user partsand application parts of an SS7 network, residing above the MTP layers.The resulting protocol stacks are illustrated in FIG. 2.

In order to provide the INAP and MAP (as well as certain otherapplication parts, e.g. OMAP, which also rely upon connectionlesscommunication over the signalling network) with certain functions andprotocols as well as a standard and common interface between theapplication parts and the network signalling service, a TransactionCapabilities Application Part (TCAP or TC) is interposed between theseapplication parts and the MTP. Furthermore, a Signalling Connection andControl Part (SCCP) is interposed between the TCAP and the MTP forcontrolling the signalling connection. The SCCP is also used by certainother application parts (referred to as SCCP users, e.g. BSSMAP) whichdo not use the services of the TCAP and which rely uponconnection-oriented and/or connectionless communication over thesignalling network.

SS7 makes use of addresses known as Destination Point Codes (DPCs) toroute signalling data through the “visibility area” of atelecommunications network, the visibility area typically being thenetwork itself together with the interfaces between the network and“foreign” networks under the control of other operators. A DPC is placedin the header of an MSU and is examined by a network signalling point(SP) upon receipt of the MSU to determine the next hop for the MSU enroute to its destination. So-called Subsystem Numbers (SSNs) are used bySS7 to direct data to specific application parts (see below) and arealso included in MSUs.

With reference to FIG. 2, it is noted that routing between variousapplication parts above the SCCP layer is achieved using so-called“global titles”. A global title contains (amongst other things) a numberdialled in the PLMN or the number of a roaming mobile station (for thespecific example shown in FIG. 1). The SCCP contains all the network androuting information required to analyse a global title and translate itinto a DPC and, optionally, a Subsystem Number (SSN) which identify thenext or final signalling point in the SS7 network.

In an SS7 network, any change in the DPC allocation within thevisibility area requires the operator to update the DPC database (orrouting table) which exists in each SP of the network. This however addssignificantly to the maintenance overheads of the network. The dedicatednature of SS7 makes it in general expensive to install and maintain (inrelation to both hardware and software), a significant barrierespecially to prospective new telecom operators. Furthermore, as an SS7network occupies bandwidth on TDMA frames of the E.1/T.1 transmissionprotocols (one slot per time frame), the bandwidth available for actualuser call data is restricted. Yet another disadvantage of traditionalsignalling architectures is that the interoperability of SS7 networks islimited due to the dedicated nature of the MTP physical layers.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to overcome or at leastmitigate the above noted disadvantages of existing telecommunicationsignalling systems.

According to a first aspect of the present invention there is provided amethod of transmitting signalling information in a telecommunicationsnetwork between a pair of application parts, the method comprising;

generating said signalling information at a first of said applicationparts, including addressing information associated with the second ofthe application parts;

passing said signalling information from the application part to aTransaction Capabilities Application Part (TCAP);

passing said signalling information from the TCAP to an adaptation layerarranged to determine an IP address and port number associated with saidaddressing information;

passing said signalling information and said IP address and port numberto an Internet Protocol (IP) part and transmitting the signallinginformation over an IP network to said destination IP address in one ormore IP datagrams;

decapsulating said signalling information at the destination associatedwith the IP address and at an adaptation layer identified by said portnumber; and

routing the signalling information to the second user part.

The use of the IP based network for transmitting signalling informationreduces the need for conventional signalling infrastructure (althoughthis may still be used in part). IP based networks offer increasedflexibility (e.g. routers of the network have self-updating routingtables) and reduced operating, maintaining, and engineering costs incomparison with conventional telecommunications signalling networks.

It will be appreciated that the Signalling Connection and Control Part(SCCP), which is normally interposed between the TCAP and the MTP in anSS7 network, is omitted from the present invention in so far as itrelates transmitting signalling information from the TCAP to an IP part.Rather, the adaptation layer interfaces the TCAP to the IP part. SCCP isdesigned to segment signalling messages into 272 octet blocks, such asare required by the MTP of SS7. In contrast, individual TCP and UDPdatagrams are capable of carrying a much larger number of octets than anMTP message. Thus, by omitting the SCCP, the present invention avoidsunnecessary segmentation of signalling messages.

The method of the present invention is particularly applicable to PublicLand Mobile Networks (PLMN), where said pair of application parts maybe, for example, peer Mobile Application Parts (MAPs) or IntelligentNetwork Application Parts (INAPs) present at respective signalling nodesof the PLMN.

In certain embodiments of the invention, the adaptation layer performs atranslation between a Destination Point Code (DPC) and an associated IPaddress and port number. In other embodiments however, the IP addressand port number are obtained at the adaptation layer directly from aglobal title provided as part of the addressing information from theapplication part.

Preferably, the adaptation layer is arranged to provide all servicefunctionality that the TCAP would otherwise receive from a SCCPincluding sequence control, return on error, and congestion control.

Preferably, the adaptation layer is arranged to encapsulate signallingand addressing information into a form suitable for further processingby the IP part.

Preferably, the signalling information is passed from the adaptationlayer to the IP part via a UDP or TPC encapsulating layer. Morepreferably, the adaptation layer is arranged to encapsulate signallingand addressing information into a form suitable for further processingby the UDP or TCP layers.

According to a second aspect of the present invention there is providedapparatus for transmitting signalling information in atelecommunications network between a pair of application parts, theapparatus comprising;

generating means for generating said signalling information at a firstof said application parts, including addressing information associatedwith the second of the application parts;

a Transaction Capabilities Application Part (TCAP) for receiving saidsignalling information;

an adaptation layer arranged to receive said signalling information fromthe TCAP and to determine an IP address and port number associated withsaid addressing information;

an Internet Protocol (IP) part arranged to receive said signallinginformation and said IP address and port number and to transmit thesignalling information over an IP network to said destination IP addressin one or more IP datagrams;

processing means at the destination associated with the IP address andat an adaptation layer identified by the port number, arranged todecapsulate said signalling information; and

routing means arranged to route the signalling information to the seconduser part.

According to a third aspect of the present invention there is provided agateway node for coupling signalling information from a common channelsignalling (CCS) network of a telecommunications system, to an InternetProtocol (IP) based network, the gateway node comprising:

a Message Transfer Part (MTP) arranged to receive signalling informationover the CCS network;

a Signalling Connection Control Part (SCCP) arranged to receive saidsignalling information from the MTP whilst controlling the signallingconnection over the CCS network;

an adaptation layer arranged to receive said signalling information fromthe SCCP and to determine an IP address and port number associated witha global title or Destination Port Code (DPC) included in the signallinginformation;

an IP layer for receiving the signalling information and the IP addressand port number from the adaptation layer and for arranging fortransmission of the signalling information over the IP based network inthe form of IP datagrams.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and in order to showhow the same may be carried into effect reference will now be made, byway of example, to the accompanying drawings, in which:

FIG. 1 illustrates schematically a GSM network together with thesignalling protocols used therein;

FIG. 2 illustrates schematically the signal processing layers of an SS7protocol of the network of FIG. 1;

FIG. 3 illustrates schematically the use of a TCP/IP network to transmitsignalling information in a telecommunications network; and

FIG. 4 illustrates signalling protocol layers present at nodes of thetelecommunications network of FIG. 3.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The general structure in terms of signalling nodes and signallinginterfaces of a GSM PLMN has already been described above with referenceto FIGS. 1 and 2. FIG. 3 shows various nodes of the GSM PLMN where it isassumed, for the purpose of this discussion, that each of these nodescomprises an application part (e.g. MAP, INAP) having a conventionalcomposition such that it can be supported by SS7 for the sending ofsignalling information to a peer application part at some other node.Considering in particular the MSCs 3 a,3 b and the GMSC 1, these nodesare connected to a conventional SS7 network 11 for the purpose ofcommunicating signalling information.

Intelligent network and PLMN specific nodes, including the IN node 6,VLR 7, HLR 8, AUC 9, and EIR 10, each have application parts (and inparticular MAP and INAP) similarly constructed for peer-to-peercommunication via SS7. However, these nodes each comprise an additionalinterface which enables them to communicate signalling information viaan IP network 12. In order to enable interworking of the SS7 connectednodes and the IP network connected nodes, the two signalling networks11,12 are connected by a gateway node 13.

FIG. 4 shows schematically the protocol layers present at a nodeconnected to the IP network 12 (Node 1), at the gateway node 13 (Node2), and at a node connected to the SS7 network 11 (Node 3). Of thelayers shown, the INAP/MAP, TCAP, SCCP, TCP/IP, physical layer, and MTPare substantially conventional in structure and therefore will not bedescribed in detail. Rather, the reader should make reference to theliterature including “Understanding Telecommunications”, vols. 1 & 2,Studentlitteratur, Lund, Sweden (ISBN 91-44-00214-9), and the ITU-T(International Telecommunications Union-Technical recommendationsstarting with Q.700 which define Signalling System Number 7 (SS7). Thepresent discussion is concerned in the main with the Adaptation layerwhich enables the TCP/IP and physical layers to replace the MTP (andSCCP), such that the TCAP can communicate over the IP network 12.

As has already been discussed above, MSUs are routed in an SS7 networkusing Destination Point Codes (DPCs) which are attached to each of theMSUs. Within a signalling point, messages are directed to a specificapplication (e.g. HLR, VLR, etc) on the basis of a Subsystem Number(SSN) also attached to the MSU. On the other hand, datagrams in an IPnetwork are routed to a destination machine on the basis of an IPaddress, and to an application running on a particular machine on thebasis of an IP port number. A main function of the Adaptation layertherefore is to translate between global titles and IP addresses andport numbers, a process which is achieved using a database associatingglobal titles and IP addresses/port numbers.

At Node 1, for signalling data to be transmitted to Node 3, theadaptation layer determines an IP address and port number associatedwith a global title contained in the message “primitive” received fromthe TCAP. The Adaptation layer then encapsulates the primitives into aformat which is acceptable to the TCP (or UDP) layer before passing theinformation to the TCP (or UDP) layers. A further function of theAdaptation layer is to monitor the state of the connection between thetwo communicating peer application parts. For example, if communicationbetween the application parts breaks down, and cannot be re-established,then the Adaptation layer may report this to the TCAP. It will beappreciated that the Adaptation layer communicates with the TCAP in amanner identical to that of the SCCP (which is omitted).

The TCP/UDP layers pass the encapsulated signalling information to theIP layer. The messages transmitted over the IP network 12 have thefollowing structure:

Physical IP TCP Addresses TCAP INAP/MAP header header header Globalheader data title/DPC/ SSN

At the gateway node 13 (Node 2 in FIG. 4), the Adaptation layer liesabove the IP network and SS7 network side layers. The adaptation layerprovides for decapsulation of the received message and passes it to theSCCP on the SS7 network side for further processing.

The messages received by the SCCP from the adaptation layer containeither a global title or a DPC/SSN or both. In the case that the messagecontains only a global title, then the SCCP translates that titles intoa DPC/SSN as already described above. For transmission over the SS7network 11, the SCCP passes the primitives, with associated DPCs, to theMTP which handles transmission in a known manner. Similarly, theprotocol layers at the receiving node (Node 3) correspond to theconventional SS7 structure.

It will be appreciated by the skilled person that various modificationsmay be made to the above described embodiment without departing from thescope of the present invention. For example, the connectivity of thenetwork nodes shown in FIG. 3 may be varied, such that certain IN nodesare connected to the SS7 network, whilst the MSCs and/or the GMSC may beconnected to the IP network. It may also be the case that nodes areconnected to both the SS7 network and the IP network, such that onenetwork provides a back-up for the other network.

It will also be appreciated that in the event that the TCAP providesaddressing information to the adaptation layer which includes a DPC/SSN,rather than just a global title, then the adaptation layer may translatebetween the DPC and the IP address/port number.

What is claimed is:
 1. A method of transmitting signalling informationin a telecommunications network between a pair of application parts, themethod comprising; generating said signalling information at a first ofsaid application parts, including addressing information associated withthe second of the application parts; passing said signalling informationfrom the application part to a Transaction Capabilities Application Part(TCAP); passing said signalling information from the TCAP to anadaptation layer arranged to determine an IP address and port numberassociated with said addressing information; passing said signallinginformation and said IP address and port number to an Internet Protocol(IP) part and transmitting the signalling information over an IP networkto said destination IP address in one or more IP datagrams;decapsulating said signalling information at the destination associatedwith the IP address and at an adaptation layer identified by said portnumber; and routing the signalling information to the second user part.2. A method according to claim 1 for use in a Public Land Mobile Network(PLMN), where said pair of application parts are peer Mobile ApplicationParts (MAPs) or Intelligent Network Application Parts (INAPs) present atrespective signalling nodes of the PLMN.
 3. A method according to claim1 and comprising passing addressing information from the TCAP to theadaptation layer which includes a global title.
 4. A method according toclaim 1 and comprising passing addressing information from the TCAP tothe adaptation layer which includes a Destination Point Code andoptionally a Subsystem Number.
 5. A method according to claim 1, whereinthe adaptation layer provides all service functionality that the TCAPwould otherwise receive from a SCCP including sequence control, returnon error, and congestion control.
 6. A method according to claim 1 andcomprising passing the signalling information from the application partto the adaptation layer via a Transaction Capabilities Application Part.7. A method according to claim 1, wherein the adaptation layerencapsulates signalling and addressing information into a form suitablefor further processing by a UDP/TCP layer disposed between theadaptation layer and the IP part.
 8. Apparatus for transmittingsignalling information in a telecommunications network between a pair ofapplication parts, the apparatus comprising: generating means forgenerating said signalling information at a first of said applicationparts, including addressing information associated with the second ofthe application parts; a Transaction Capabilities Application Part(TCAP) for receiving said signalling information; an adaptation layerarranged to receive said signalling information from the TCAP and todetermine an IP address and port number associated with said addressinginformation. an Internet Protocol (IP) part arranged to receive saidsignalling information and said IP address and port number and totransmit the signalling information over an IP network to saiddestination IP address in one or more IP datagrams; processing means atthe destination associated with the IP address and at an adaptationlayer identified by the port number, arranged to decapsulate saidsignaling information; and routing means arranged to route thesignalling information to the second user part.
 9. A gateway node forcoupling signalling information from a common channel signalling (CCS)network of a telecommunications system, to an Internet Protocol (IP)based network, the gateway node comprising: a Message Transfer Part(MTP) arranged to receive signalling information over the COS network; aSignalling Connection Control Part (SCCP) arranged to receive saidsignalling information from the MTP whilst controlling the signallingconnection over the CCS network; an adaptation layer arranged to receivesaid signalling information from the SCCP and to determine an IP addressand port number associated with a global title or Destination Port Code(DPC) included in the signalling Information; an IP layer for receivingthe signalling information and the IP address and port number from theadaptation layer and for arranging for transmission of the signallinginformation over the IP based network in the form of IP datagrams.